Because of the need to reduce fuel emissions, clean-burning middle distillate fuel blends such as diesel and jet fuels are required. These distillate fuel blends can come from many sources, including Fischer-Tropsch synthesis, olefin oligomerization, hydrotreating and hydrocracking processes and combinations thereof. One existing method of creating cleaner fuels involves the severe hydroisomerization of distillate streams to form highly isoparaffinic products. In severely hydroisomerized materials, the lubricity is often low due to the absence of oxygenates, aromatics and heterocyclics such as sulfur-containing compounds. However, lubricity is required for the efficient operation of fuel delivery systems, and the need exists for an environmentally benign way of increasing the lubricity of clean distillate fuel blends. Low lubricity can lead to excessive wear of metal engine parts which can result in poor engine performance, additional pollution, lower fuel mileage and early replacement of engine parts.
Fuel lubricity can be measured by one of two methods: ASTM D6078 or ASTM D6079. ASTM D6078 is a scuffing load ball-on-cylinder lubricity evaluator method (SLBOCLE) and D6079 is a high frequency reciprocating rig method (HFRR). Fuels having a D6078 lubricity of less than about 2,000 grams are not likely to prevent excessive wear in injection equipment, while fuels with values above about 3,100 grams should provide sufficient lubricity in all cases. If D6079 is used, fuels with values above about 600 microns are not likely to prevent excessive wear while fuels with values below about 450 microns should provide sufficient lubricity in all cases.
The reproducibility limit for ASTM D6078 is ±900 grams, and the reproducibility limit for ASTM D6079 is ±80 microns. Thus, an increase in the D6078 value of about 900 grams or more or a decrease in the D6079 value of about 80 microns or more demonstrates an absolute improvement in lubricity. However, D6078 increases of about 225 grams or D6079 decreases of about 20 microns or more provide an acceptable measure of a fuel with improved lubricity provided that the measurements are made on the same equipment and a sufficient number of measurements are made.
Higher alkyl alcohols, i.e., alcohols with at least four carbons, are known to improve the lubricity and stability of distillate fuel blends. These higher alcohols are also used in detergents and plasticizers, and the selling price for such alcohols is often more than one dollar per kilogram, significantly more than the price for fuels. Thus, there is a strong economic incentive to recover the alcohols present in distillate fuel blends rather than allow them to be converted into fuels.
Some primary higher alkyl alcohols are created during the normal production of distillate fuel blends. For example, in a Fischer-Tropsch synthesis, a synthetic gas composed mostly of CO and H2 is reacted in the presence of a catalyst to form a wide range of gaseous and liquid hydrocarbon products including a paraffinic wax. Alcohols and olefins are normally present in some Fischer-Tropsch products. However, the amount of alcohols normally produced during a Fischer-Tropsch synthesis is not enough to raise the lubricity of the isoparaffinic distillate fuel blends to an acceptable level.
The Fischer-Tropsch synthesis provides a light naphtha stream containing low molecular weight linear paraffins and olefins as well as oxygenated compounds such as alcohols. These naphthas are too volatile for incorporation into distillate transportation fuels and their levels of olefins and oxygenates makes them unsuitable for use in gasoline or as a petrochemical plant feed. They are considered less valuable than distillate fuels but it is not possible to vary reaction conditions to selectively eliminate production of light naphthas and increase production of distillate transportation fuels. In conventional practice, the light naphthas must be further refined to reduce the content of olefins and oxygenates in order to provide a salable naphtha.
The use of alcohols, and specifically higher primary alkyl alcohols, to improve lubricity in highly isoparaffinic distillate fuel blends has been disclosed in a variety of patents. U.S. Pat. No. 5,814,109 describes a process for producing higher alcohol-containing additives for distillate fuel blends through a Fischer-Tropsch reaction. These additives are useful for improving the cetane number, lubricity, or both, of a mid-distillate diesel fuel. U.S. Pat. No. 5,766,274 describes the production of a clean jet fuel distillate by hydroisomerizing only the heavier portion of a Fischer-Tropsch wax and blending with the lighter portion which retains most of the alcohols in the Fischer-Tropsch wax product. Because most of the alcohols are retained, the blended distillate has a higher lubricity than if the entire wax were hydrotreated. U.S. Pat. No. 5,689,031 describes the production of a clean distillate, for use as a diesel fuel, by hydroisomerizing a portion of a Fischer-Tropsch wax. Again, because only a portion of the wax is hydrotreated, some of the alcohols are preserved in the untreated portion thereby increasing the lubricity of the distillate. WO 01/46347 A1 discloses significantly improved reduced particulate emission performance of exhausts of vehicles powered by fuel combustion both at high and low loads by adding oxygenates or other hydrocarbon components in a diesel fuel composition comprising a major amount of a base fuel and a relatively minor amount of at least one chemical component other than that generated in a refinery process stream. WO 01/46348 discloses a fuel composition comprising a base fuel having 50 ppm or less or sulfur, 10% or less of olefin, 10% or less of ester and at least 1 wt. % of oxygenate chosen from certain alcohols(s) and ketone(s) and having no other oxygen atom in its structure, with improved reduction of particulate emission without using further additives such as cyclohexane or peroxides or aromatic alcohol and with little to no increase in nitrogen oxide (NOx) emission at high engine loads.
However, in each of these processes, olefins are retained in addition to the alcohols. Frequently, the olefins outnumber the alcohols on a molar basis. The presence of olefins is undesirable and can lead to thermal stability problems, and also to the formation of gums. A need exists for a distillate diesel or jet fuel blend with improved lubricity through the use of alcohols, but without the inclusion of olefins in the final product. A low level of olefins is defined as a molar ratio of total olefin to total alcohol below 0.5, preferably below 0.25 and most preferably below 0.1. Ideally, the olefin content should be below the limit of detection.
In addition, a need exists for methods of recovering alcohols from a distillate fuel product and methods of removing unwanted olefins without the use of hydrogen, which is expensive.
Techniques are well known in the art for converting olefins to ethers, and then to alcohols. For example, formation of di-isopropyl ether from alcohols and olefins is well known. U.S. Pat. No. 5,405,814 describes the conversion of light olefins, especially propylene, to a mixture of alcohols and ethers. U.S. Pat. No. 4,962,239 discloses the synthesis of ethers from alcohols and olefins using Zeolite MCM-22. U.S. Pat. No. 5,231,233 discloses the synthesis of ethers and/or alcohols using Zeolite MCM-36. U.S. Pat. No. 5,285,560 discloses a catalytic distillation reactor instead of a fixed bed reactor for synthesizing ethers. The disclosures of these patents are incorporated herein by reference. However, these processes focus on the production of lower alkyl ethers that are more volatile than their starting alcohols or olefins, so the ethers could not be easily separated from the mixture by distillation. Also, these processes do not address lubricity concerns or olefin content.
Often, the ethers and alcohols formed by known processes are of relatively low molecular weight, and sometimes environmentally harmful. For example, the dangers of methyl tertiary butyl ether (MTBE) and methyl tertiary amyl ether (TAME) when used as blend components in gasoline are well known. MTBE and TAME contaminate underground aquifers when gasoline spills or leaks. MTBE and TAME have significant water solubility, and eventually hydrolyze to form tertiary butyl alcohol and tertiary amyl alcohol. Because these alcohols and their parent ethers lack protons on the carbon in the non-methyl group adjacent to the oxygen in the ether, they are not very biodegradable. Thus, they partition into the underground water phase and remain there for a long time. The lack of reactive protons in this position of MTBE and TAME is consistent with their chemical structural identity of being “tertiary” alcohols.
It is therefore an object of the present invention to improve the lubricity of middle distillate fuels such as diesel and jet fuels by the use of higher primary and secondary alkyl alcohols while minimizing the presence of olefins, and to develop a process for making such a distillate fuel with good lubricity.
It is another object of the present invention to increase the lubricity of middle distillate fuel blends in an environmentally benign way. A further object of the invention is to produce a mixture of primary and secondary alcohols as a pure product substantially free of hydrocarbons. Yet another object of the invention is to convert the olefins in Fischer-Tropsch naphtha and distillate products without the use of hydrogen.
These and other objects of the present invention will become apparent to the skilled artisan upon a review of the following description and the claims appended thereto.